In the field of printing labels or receipts, it is common practice to use thermal printer devices that print on paper strip having a face that is heat-sensitive.
Each label or receipt forming a printed piece of paper is taken from a roll of blank paper which is generally housed in a compartment of the equipment. Access to the compartment for changing the roll takes place via an opening that is closed by a moving wall which carries a portion of the printer, generally the backing (and drive) roller that engages the paper beneath the print head, thereby making it easier to load the roll in the printer and more particularly to put its leader into position.
Various devices exist that implement a printer which opens. Some of them have a print head (in the form of a ceramic plate with a line of heater points) pivotally mounted on a stationary frame and associated with a spring tending to apply return torque to the plate about its pivot axis, urging it towards the backing roller (sometimes referred to as the xe2x80x9ccapstanxe2x80x9d). This return torque makes it possible firstly for the head to retract while the cover carrying it moves in an opening or a closing direction, and secondly for the cover to be locked in the closed state, with the plate bearing against the capstan, holding onto the capstan resiliently and thus holding shut the cover to which the capstan is attached.
The main drawback of such devices lies in the pivot axis of the head and that of the capstan not being exactly parallel because of manufacturing tolerances, wear, etc., which factors cannot be overcome at reasonable cost. When they are not parallel, this leads to pressure on the print head (line of points) against the paper being non-uniform, which in turns leads to print quality varying across the width of the strip of heat-sensitive paper.
The present invention provides a remedy to that drawback by ensuring better positioning between the capstan and the print head at the time of printing, and doing so in a manner that is simple and of low cost, thereby improving the quality of the printing along an entire line of heater points.
The invention thus provides a thermal printer device comprising a stationary frame having a print head provided with a line of heater points and a moving frame that carries a backing roller the paper strip via the ends of an axle and that moves relative to the stationary frame between a first relative position in which the roller has one of its generator lines in contact with the line of heater points of the print head, and a second relative position in which the roller and the print head are spaced apart from each other. In characteristic manner, the print head is fixed to the stationary frame and the device includes a holding mechanism for pressing the roller against the line of heater points when the frames are in the first relative position, the mechanism comprising respective hooks for receiving each of the ends of the axle of the roller, the hooks being slidably mounted on the stationary frame to move in a direction perpendicular to the line of heater points and being subject to the action of respective resilient return members directed towards the print head.
In the invention, by providing a print head that is absolutely stationary in the stationary frame, all uncertainty as to the position of the head relative to that frame is eliminated, where such uncertainty exists by construction in the pivoting heads of the prior art because of manufacturing tolerances for the hinge (frames are generally made of plastics material) and because of play that vary with equipment wear. Then, by providing for the roller (capstan) to be pressed against the line of points by mutually independent hooks at each end of the axle of the roller, the roller is pressed properly and in balanced manner against the line of heater points, unlike previously known devices in which the pressure exerted by the head on the roller forces the roller into support bearings which define the orientation of the roller independently of the orientation of the head.
To enable the hooks to take charge of the roller in this way without stress, and assuming that the moving frame is in the form of a cover hinged to the stationary frame about an axis parallel to the line of heater points, the hinge between the cover and the stationary frame is made to include sufficient play to allow the hooks to compensate for lack of parallelism between the line of points and the hinge of the cover.
In preferred manner, and so as to ensure that the roller has maximum freedom of positioning relative to the moving frame that carries it, each end of the axle of the roller is carried by the moving frame in a slot that is substantially parallel to the sliding direction of the hook when the moving frame is in the above-mentioned first relative position.
In an advantageous embodiment of the device of the invention in which the print head comprises a ceramic plate associated with a metal radiator, the stationary frame is formed by the radiator placed as a spacer between two angle supports each having a sliding guide for a respective one of the hooks extending substantially perpendicularly to the radiator.
This simple structure makes it possible with a minimum number of identical parts to cover an entire range of printer devices that differ only in the width of the strip of paper that is to be printed.